Viscosity index, light stability, aromatics content and iodine number are measurements which are employed in lubricating oil specifications as general indicators of the quality of the oil. Viscosity index is a reflection of an oil's resistance to viscosity change with fluctuations in temperature. The higher the viscosity index of an oil, the more resistant it is to a viscosity change caused by temperature fluctuation. Iodine number is an indicator of the amount of unsaturated linkages appearing in the molecules present in the oil to which iodine can be added. Unsaturated linkages generally are undesirable because such linkages are more readily oxidized than saturated linkages, especially at elevated temperatures, and such oxidation results in degradation of the oil. Therefore, a high quality lubricating oil, i.e., one that is particularly desirable for automotive uses, should possess a relatively high viscosity index and a relatively low iodine number.
The stability of oils also is affected by the presence of aromatic materials in the oil. The aromatic content of oils can be reduced by hydrotreating or hydrocracking. Hydrotreating processes are preferred where it is desired to reduce aromatics without significantly increasing the amount of undesirable low boiling materials. Hydrotreating processes can be effective for the saturation of aromatic compounds to naphthenic materials without significant cracking or hydrocracking.
The upgrading of crude lubricating oil stocks by means of catalytic hydrogenation has been suggested in the art. Generally, the processes require, in a first stage, the treatment of the crude lubricating oil stocks with hydrogen under conditions of elevated temperature and pressure while employing a catalyst comprising hydrogenating components (metals) supported on a carrier having a substantial degree of cracking activity such as for example, alumina, silica, and mixtures thereof. Although many of the catalysts which have been suggested in the prior art provide some improvement, there continues to be a need for improved multifunctional catalysts which provide increased yields, higher viscosity indexes and reduced aromatics content under lower reaction temperatures.
The acidic or cracking function in the hydrotreating process usually is supplied by the catalyst support or the catalyst support enhanced by acidic promoters such as halogens. The hydrogenation activity of a supported catalyst is supplied by the hydrogenation metal component which may exist in the final catalyst as a metal, the metal ion complexed with the support structure and other promoters, or metal compounds, notably the oxides and sulfides. Typical hydrogenation metals are metals of Groups VIB and VIII of the Periodic Table of Elements.
The hydrogenation catalysts which are useful in hydrotreating crude oils generally serve a multiplicity of functions such as cracking, nitrogen removal, sulfur removal, metal removal, hydrogenation, etc. Various catalysts which have been suggested in the prior art will perform these functions to different degrees, and catalyst compositions have been designed and formulated to optimize their performance with respect to one or more of such functions. For example, catalysts have been suggested which are extremely useful in increasing the viscosity index and reducing nitrogen and sulfur content, but the same catalysts may not be very effective for reducing the aromatic content of the oil. Other catalysts have been designed to provide a minimum cracking of the oil and an increase in effectiveness in removing aromatics. These are normally used in the second stage of a two-stage process after the viscosity index has been increased and nitrogen and sulfur content reduced by use of a specially designed hydrotreating catalyst. Although such procedures are effective in producing the desired result, the processes do require the use of two different catalysts thereby requiring the maintenance of inventories of two different catalysts.
The physical properties of the catalysts useful in the hydrogenation reactions, in some instances, may be as important as the catalytic activity. To be useful, the catalyst must have sufficient mechanical strength to resist crushing and/or attrition in use. Since catalytic reactions generally occur at the surface of the catalyst, it is considered desirable that the catalyst have optimal surface area and pore volume. Thus, in the preparation of the catalysts, it is important to use supports of high surface areas and pore volumes because impregnation of a support with metals fills the pores and reduces the surface area.
Because of the continuing demands placed on the lubricating oil producers for improved products, the producers typically use two or more hydrotreating stages. A catalyst generally is selected for the first stage for its ability to hydrocrack the crude feedstock which typically is a vacuum gas oil. In the second and subsequent stages, a catalyst is chosen which is capable of polishing the oil with the occurrence of mild hydrogenation and aromatics removal. In general, cracking, (i.e., formation of lower boiling materials) is undesirable in the second stage.
Numerous publications, including patents, have discussed the catalytic hydrogenation of lubricating oil stocks. U.S. Pat. Nos. 3,078,238 and 3,046,218 describe a supported nickel-tungsten catalyst which has been treated with a halogen such as fluorine to improve the hydrogenation activity of the catalyst. The resultant catalyst contains at least 0.3% fluorine, and preferably 2.5% or more of fluorine. The catalyst support is preferably a mixture of alumina and silica. In the '238 patent, the catalyst composition comprises halogenated, sulfided supported nickel and tungsten wherein the carrier material possesses cracking activity. The amount of nickel and tungsten present in the catalyst should be a total of from 5% to about 40% of the total catalyst weight, and the nickel and tungsten are present in some form of combination or mixture with sulfur. The sulfur should be present in amounts of from about 2% to about 23% of the catalyst weight. The support materials are composites of silica and alumina and the materials may contain between 1% and 99% silica although compositions containing from 5% to 90% silica are more desirable, and the most desirable composites contain 65% to 90% silica. In U.S. Pat. No. 3,046,218, the catalyst support may be natural or synthetic high silica-low alumina catalyst or silica-alumina cracking catalyst which contain up to 50% alumina.
In Table I of U.S. Pat. No. 3,078,238, various catalysts containing various support compositions are identified containing various silica to alumina ratios including a catalyst support containing 5% silica and 95% alumina. This latter catalyst is reported to have a low cracking value resulting in a lube oil product having an undesirable high iodine number and a relatively low viscosity index which is about the same as the catalyst wherein the support composition is over 99.5% by weight of alumina. Based upon the results reported in the '238 patent, the patentees concluded that the catalyst should preferably have a cracking activity on the Kellogg scale of between 60 and 80 and that the support should be high-silica support comprising from 75-85% silica and from 25% to 15% alumina.
U.S. Pat. No. 3,553,107 discloses a hydrotreating catalyst and a process for treating lubricating oil stock containing from 5% to 30% aromatics by volume whereby the aromatic content of the oil is significantly reduced. The oil then is treated with fuming sulfuric acid, neutralized with caustic, and extracted with alcohol to remove sulfonates and yield a white oil. The hydrotreating catalyst used to reduce the aromatic content in the lubricating oil stock employs Group VI and Group VIII metals on an alumina support. Preferred catalysts include combinations of nickel and tungsten in amounts of from 10% to about 30% by weight and preferably about 25%. In another preferred embodiment, the catalyst is composed of 20% nickel, 20% tungsten and 2% fluorine on alumina.
U.S Pat. No. 3,493,493 describes hydrotreating catalysts useful for enhancing lubricating oils. The catalysts comprise at least one Group VI metal and one Group VIII metal on an alumina carrier having a cracking activity index of less than about 30 and containing halogen. The total metals content of the catalysts is at least 20% by weight, and the atomic ratio of Group VIII metals to Group VI metals is in the range of from about 2.25:1 to about 6:1. The carrier employed in the invention must be alumina which has low activity for the promotion of cracking. The effectiveness of the catalysts claimed in this patent is compared to catalysts using different supports containing mixtures of alumina and silica including supports comprising 75% silica and about 25% alumina.
U.S. Pat. No. 4,427,534 describes the process for the production of a jet or diesel fuel from aromatics-containing feedstock. The process comprises contacting hydrogen and a feedstock with a presulfurized catalyst composite comprising a Group VIB metal, a Group VIII metal and a halogen impregnated on a cracking support under hydrogenation/hydrocracking conditions. The preferred carrier is a silica-alumina composite containing from about 65% to about 85% silica, preferably about 70-80% silica and 20-30% alumina.